1. Field of the Invention
The present invention concerns a suspension device, and more particularly to a suspension device for a superconducting magnet heat shield enclosure which is especially suitable for MRI devices.
2. Description of the Prior Art
MRI devices have a wide range of application. MRI devices include three basic components: the magnet part, the magnetic resonance excitation and signal detection part, and the data processing and image reconstruction part. The magnet normally includes a basic field magnet for generating a stable main magnetic field and a gradient coil for generating a fast break-make gradient magnetic field. The RF device of the excitation and signal detection part transmits RF signals into the objects to be examined and receives MR signals generated thereby. These MR signals are processed by the data processing and image reconstruction part to form MR images. For example, in medical application, the images generated by the MRI device can provide accurate visual information for diagnosis and treatment of diseases.
The basic field magnet used in the MRI device can be a normal electrical magnet, a permanent magnet or a superconducting magnet. With an intense current running through, the coil made of a superconducting material (if used) generates a powerful magnetic field. The current in the superconducting coil remains unchanged even after the impressed current is cut off; therefore the superconducting magnetic field is very stable. In order to maintain the superconducting state, the superconducting magnet must be placed in a low-temperature liquid container and immersed into a coolant such as liquid nitrogen. To reduce evaporation consumption of the coolant, a heat shield enclosure is further installed in the cylinder outside the coolant. In addition, an external vacuum shell is installed outside the heat shield enclosure.
In the actual operation of the superconducting magnet, small vibrations of various components may cause relative movement between the heat shield enclosure and the magnet, which leads to generation of an eddy current in the heat shield enclosure that disturbs the homogeneity of the magnetic field and causes streaking phenomenon in the magnetic resonance image. Factors that may cause relative movement between the heat shield enclosure and the magnet include operation of the cooling head connecting to the coolant or the gradient coil to work or floor vibration, etc. Therefore, a key factor in the design of a suspension device for a heat shield enclosure is control of the relative movement between the heat shield enclosure and the magnet.
Japanese patent 61-115307 discloses a suspension system used in an MRI device. As shown in FIGS. 1 and 2 herein, the MRI device has a magnet 1, a low-temperature container 2, a heat shield enclosure and a vacuum jacket. The low-temperature container 2 that encompasses the magnet 1 cools the magnet 1. The heat shield enclosure encompassing the low-temperature container 2 has an outer heat shield layer 3a and an inner heat shield layer 3b. The vacuum jacket encompassing the heat shield enclosure also has an outer vacuum shell 4a and an inner vacuum shell 4b. 
In this system, the outer vacuum shell 4a and the outer heat shield layer 3a are connected by the connecting part 8, and the outer vacuum shell 4a and the low-temperature container 2 are connected by the connecting part 9. The position of the low-temperature container 2 relative to the magnet 1 is fixed. As shown in FIG. 2, the connecting parts are uniformly and symmetrically distributed along the diameter of the magnet 1. FIG. 3 is a schematic diagram showing the structure of a connecting part 8, which is of a rigid structure.
In such a structure, since both the low-temperature container 2 and the outer heat shield layer 3a connect with the outer vacuum shell, it is not easy to control the relative position between the low-temperature container 2 and the outer heat shield layer 3a to avoid the relative movement between the magnet and the heat shield enclosure installed in the low-temperature container 2. As a result, the problem of streaking phenomenon cannot be fundamentally alleviated.
Moreover, since the connecting parts 8 between the outer vacuum shell 4a and the outer heat shield layer 3a are rigid components distributed radially along the outer heat shield layer 3a, the outer vacuum shell 4a and the outer heat shield layer 3a have a low ability to adapt to each other in the event of any vibration generated, which is not good for absorbing and reducing any relative position displacements due to such vibration.
Therefore, there is a need to provide a suspension device for the superconducting magnet heat shield enclosure that can overcome the aforementioned shortcomings.